1. Field of Invention
This invention relates to a method and device for non-destructive testing of details, machine units and mechanisms, various materials, and in particular to a method and device for non-destructive determination of residual stresses which is based on optical holographic interferometry technique.
2. Background
Optical holographic interferometry technique is well suited for non-destructive testing of internal defects in blocks and units of machines and devices, welded seams, as well as measuring stresses of an object during the object""s work load and residual stresses caused by technological processes of welding, forging, soldering etc. These applications are useful for fields such as offshore oil industry, shipping industry, process industry, air industry, and all types of constructions where strength is vital or fatigue may cause a problem.
An example of the state of the art for measuring residual stresses in an object by holographic interferometry is given in the journal: xe2x80x9cWelding Engineeringxe2x80x9d 1983, vol. 12, p. 26-28. The article describes a typical device and method for measuring residual stresses which is based on drilling a small and shallow hole in the object for release of stresses as well as holographic interferometry technique for determination of surface displacements in the object at the edge of and in the vicinity of the drilled out hole. The principle of the method can be described as follows: First, a hologram of the investigation area of the object which is it""s initial state is recorded and developed on a registering medium. Next, the residual stresses in a point of the investigation area of the object is released by drilling a small hole in the object. Then the registering medium with the developed image of the investigation area in the initial state and the investigation area of the object with the drilled out hole are simultaneously illuminated by the reference and object beams respectively. The components of the residual stresses is determined from the interference pattern which occurs in the hologram.
This device and its operation stages are shown schematically in FIGS. 1-3. The means for formation and registration of holograms from an area of the object (10), as well as for formation of an interferogram from this area after release of residual stresses at the investigation point (14) of the object, is given schematically as an optical block with reference number (1) in FIGS. 1 and 3. The block contains a coherent light source (2), a holographic interferometer with optical elements (3-4) forming a reference beam (5) and an object (6) beam, and a recording medium (7). All components are rigidly connected with regard to each other. The optical block contains also a response part (8) of a precision device for fine positioning of the optical block on the object (10) above the area which is to be investigated (a corresponding receiving part (9) of the precision device is fastened on the object (10)). Means for drilling a hole at the investigation point (14) is given schematically as a mechanical block (11) in FIG. 2. Typical dimensions of the hole 1-3 mm in diameter and the depth are up to 1.5-2.0 times the diameter. In addition there are an apparatus for displaying and observation of the interferograms (in this case, a TV-camera (12) and a display screen (13)).
The operation of the device can be divided into three stages. The first stage is the registration of the hologram from the investigation area of the object; the second stage is the release of the residual stresses in the investigation point of the studied area of the object; the third stage is the formation of the interferogram from the studied object area and the determination of residual stresses in the point of the studied area. Let us consider the device operation stage by stage.
The First Stage
First, the receiving part (9) of the precision device is fixed on the investigation area of the object (10) (see FIG. 1). Then, the optical block (1) is installed above the investigation area of the object (10) by attaching the response part (8) of the precision device into the receiving part (9), and a hologram from the investigation area is registered. This is made in the following way:
The beam from the coherent light source (2) is expanded by the micro-lens (3). One part of the expanded beam is reflected by the mirror (4) towards the recording medium (7), this part is usually named the reference beam (5). The other part of the expanded beam hits the investigation area (14) of the object and reflects therefrom towards the recording medium (7). This part is named the object beam (6). When the object beam meets the reference beam, an interference occurs and a holographic image of the studied area of the object is formed. This image is registered and developed by means of the recording medium (7).
After development of the holographic image, a hologram of the studied area can be restored (i.e. the light wave scattered from the investigation area of the object is restored behind the recording medium (7)). For this purpose, it is necessary to illuminate the registering medium (7) (which contains the developed holographic image) with the reference beam (5).
The optical scheme is designated in such a manner that it has maximal sensitivity towards normal displacements of the surface of the object.
After finishing the holographic image registration of the studied area, the optical block (1) is removed from the object surface.
The Second Stage
The mechanical block (11) is installed on the studied area of the object (see FIG. 2) and, with its use, a small and shallow hole is drilled at the investigation point (14) of the object (10). The surface of the studied area is deformed in the vicinity of the hole due to release of residual stresses nearby the hole edge, and the normal component of the surface displacement at the hole edge is measured.
The Third Stage
First, the optical block (1) is extremely precisely reinstalled in the original position which it had at the first stage of the measurements (see FIG. 3) by using the precision device (8, 9). The error of positioning should be less than one wavelength. Then, the illumination of the recording medium (with the developed holographic image of the studied object area in its initial state) by the reference beam (5), and illumination of the studied area with the drilled out hole by the object beam (6) are performed simultaneously.
Thus, two light waves scattered from the investigation area of the object will simultaneously be behind the recording medium (7). One of which corresponds to the light wave scattered by studied area of the object in its initial state (before drilling the hole), and the other to the light wave scattered by the studied area of the object with the drilled out hole. As a result of the interference of these light waves, an interferogram (15) of the studied area is formed (see FIG. 3) which can be observed, for example, with a TV-camera (12) and displayed by suitable means (13). From the interferogram one can determine the normal components of the surface displacement at the hole edge. In any considered direction, for example, along the X-axis, the normal component of the surface displacement (Wx) at the hole edge will be equal to the number of interference fringes (N) (observed in the chosen direction), multiplied by one half of the wavelength (L) and divided by the sine of the incidence angle of the object beam (6). The residual stresses can be calculated by using the measured values of the normal component of the displacement at the hole edge. This may be performed in the following way.
In the case of a welded seam, for instance of an aluminium plate, the main residual stresses Qxx, Qyy are directed in parallel and perpendicular to the welded seam, respectively, and the interference pattern consists of two pairs of mutually perpendicular lobes (15) which are schematically presented on the display screen (13) (see FIG. 3). In this case, the main stresses Qxx and Qyy are determined from simplified theoretical expressions (1) and (2) by using experimentally measured normal components of the surface displacements at the hole edge, Wx and Wy, and assuming that the depth of the drilled out hole (hs) is less or equal to its radius (r3):                               Q          x                =                                                            W                x                                            W                                  1                  ⁢                  x                                                      ⁡                          [                                                r                  1                                /                                  r                  s                                            ]                                ⁢                      {                          E              /                              E                AL                                      }                                              (        1        )                                          Q          y                =                                                            W                y                                            W                                  2                  ⁢                  x                                                      ⁡                          [                                                r                  1                                /                                  r                  s                                            ]                                ⁢                      {                          E              /                              E                AL                                      }                                              (        2        )            
where W1x, W2x are parameters equal to the normal components of the surface displacement at the hole edge along the X-axis for unity values of stresses applied first in the X-axis direction (when determining W1x) and, then in the Y-axis direction (when determining W2x), and which is obtained from the theoretical dependencies of W2x, W1x on the ratio between rs and hs under unity stress for the studied material. E and EAL are elasticity modules of the studied material and aluminium, respectively.
However, the above mentioned method and equipment for determining residual stresses have essential drawbacks:
1) It is necessary to drill holes in the object that is to be investigated for residual stresses. Thus the method is a destructive test, and is obviously not acceptable for a variety of objects and applications.
2) It is necessary to remove the optical block with holographic interferometer from the studied area of the object before drilling out the hole, and to reinstall it with extreme precision in its original position. On one hand, this considerably increases the time consumption of measurements, thus the evaluation of residual stresses is not performed in a real-time scale. And on the other hand, this requires the use of extremely fine tuned precision devices for positioning of the optical block on the studied area of the object.
An attempt to eliminate the mentioned drawbacks was made in a device for measuring residual stress, described in U.S. Pat. No. 5,432,595 to Pechersky. The device and its operation stages are, schematically given in FIGS. 4-6. From the figures one see that the device has a similar optical block as the device described above (FIGS. 1-3), but the mechanical block is substituted with a pulse source (16) of infrared radiation (IR) and a mirror (17) to direct the IR-pulse to the chosen investigation point on the object.
The operation of the device also consists of three stages, namely the registration of a hologram from the investigation area of the object (see FIG. 4), followed by the release of residual stress in the investigation point (see FIG. 5), and finally a formation of the interferogram from the studied area (FIG. 6). In this case the release of the residual stresses is achieved by heating the investigation point by radiating it with the IR-pulse until it reaches the temperature of material transition into the plastic state. In contrast to the above mentioned method, this eliminates the need for removing the holographic block (1) between the registering of the holograms at stage one and three, and consequently obtains an interferogram of the investigation area practically instantaneously after performing the IR-irradiation of the investigation point, i.e. in real-time scale.
However, this device and method also suffer from considerable drawbacks which can be summarised as follows:
1) Deviation of the energy distribution over the area of the IR-pulse from a rectangular shape as well as the heat dissipation from the investigation point of the object irradiated with the IR-pulse results in a blurring out of the boundaries of the spot where the release of residual stresses occurs. This excludes the use of expressions (1) and (2) for quantitative evaluation of residual stresses from the results on measurement of normal components of surface displacement. It also makes it problematic to obtain analytical expressions for subsequent quantitative determination of residual stress from the measurement of normal components of surface displacement, and makes the assignment of determined residual stress to particular point of the object difficult.
2) Due to the heating of the investigation point up to its transition temperature to the plastic state where the residual stresses are released, the action of residual stresses localised outside of the heated spot will deform the surface of the object not only in the vicinity of the heated spot but also within the spot itself. This is an additional confirmation for the above given conclusion that this device does not allow the use of the analytical expressions given in equations (1) and (2), since these assumes that the stress release occurs in a spot with sharp boundaries and no deformation within the region with released stresses. Further, the problem of obtaining new analytical expressions for quantitative determination of residual stresses is very complicated due to uncertainty in the determination of the boundaries of the region of stress release, the transition of the material into the plastic state in the region of stress release, and the deformation of the region of stress release. This allows one to assume that the considered device can only be used, at best, to reveal residual stresses.
3) Structural changes in the irradiated spot occurs during heating up to the transition temperature by the IR-pulse create new stresses. These new stresses together with residual stresses localised outside the region of residual stress release, should deform the irradiated region and its surroundings as well. Therefore it becomes impossible, from the distribution of normal displacement components outside the irradiated spot, not only to quantitatively determine the residual stresses, but even to determine the directions of the main residual stresses. For example, in the case of a welded seam and the previously described device for determining residual stresses, the directions of the lobes of the interference pattern (15) (see FIG. 3) correspond to directions of main residual stresses. In the present device, the interference pattern (around the region of residual stress release of the welded seam) is very complicated and differs completely from that presented in FIG. 3, thus making it practically impossible to determine the directions of the main residual stresses.
The main object of the invention is to provide a device and method for performing non-destructive real-time determinations of residual stresses in materials by holographic interferometry which overcomes the above mentioned drawbacks.
It is also an object of the invention to provide a device and method for performing non-destructive real-time determinations of residual stresses in materials by holographic interferometry which is able to release the residual stresses in a region with a sharp boundary of the object.
It is also an object of the invention to provide a device and method for performing non-destructive real-time determinations of residual stresses in materials by holographic interferometry which makes it possible to employ the simple expressions given in equations (1) and (2) to calculate the residual stresses.
The objectives of the invention can be achieved by the device and method disclosed in the appended claims and in the description given below.
The objectives of the invention can be achieved by exposing a certain region (the investigation point) of the investigation area of the object to a xe2x80x9cdislocationxe2x80x9d release of the residual stresses. A preferred way to obtain this is by exposing the investigation point of the object to an electric high-current pulse since this enables a very fast release of the residual stresses and eliminates the need to move the optical block. During exposure to an electric pulse, an energy transfer from directionally travelling electrons to the dislocations occurs, and this phenomena as well as the magneto-dynamical effect of the percussion compression of the investigation area (in which the electron stream is passing) leads to the directional movement of dislocations and to release of residual stresses. The release of residual stresses is thus carried out without causing a transition of the material into a plastic state, and it can be done in a region with a sharp boundary. Thus, if one employs an optical block which may be similar to the optical blocks described in the prior art, but which also includes a device for release of residual stresses by delivering an electric pulse at investigation point of the surface of the object, the drawbacks of the above described devices and methods is avoided. It will also be possible to use the world-widely collected experience in calculating the residual stresses by using the analytical expressions given in equations (1) and (2), as well as results on experimental determination of normal components of the surface displacement at the boundary of the region of stress release.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.